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Eon (geology)
In general usage, an eon (sometimes spelled aeon) is a period of time arbitrarily designated by humans. Geologists refer to an eon as the largest subdivision of time on the geologic time scale. For example, the Phanerozoic Eon, which is about 550 million years long, covers the period of time during which animals with hard shells that fossilize would have been abundant. An eon is composed of several geologic eras, which in turn are composed of geologic periods, which are composed of geologic epochs. We are currently in the Phanerozoic Eon, the Cenozoic Era, the Neogene Period, and the Holocene epoch. Formerly, only one eon existed besides the Phanerozoic: the Precambrian. More recently, the Hadean, Archean, and Proterozoic "eras" of Precambrian time have been considered eons. The geologic timescale in terms of eons, eras, and periods looks like this: ImageSize = width:800 height:100 PlotArea = left:65 right:15 bottom:20 top:5 AlignBars = justify Colors = id:neogene value:rgb(0.99215,0.8,0.54) id:paleogene value:rgb(1,0.7019,0) id:cretaceous value:rgb(0.5,0.764,0.1098) id:jurassic value:rgb(0.302,0.706,0.5) id:triassic value:rgb(0.403,0.765,0.716) id:permian value:rgb(0.404,0.776,0.867) id:carboniferous value:rgb(0.6,0.741,0.855) id:devonian value:rgb(0.6,0.6,0.788) id:silurian value:rgb(0.694,0.447,0.714) id:ordovician value:rgb(0.976,0.506,0.651) id:cambrian value:rgb(0.984,0.5,0.373) id:neoproterozoic value:rgb(0.792,0.647,0.583) id:mesoproterozoic value:rgb(0.867,0.761,0.533) id:paleoproterozoic value:rgb(0.702,0.698,0.369) id:eoarchean value:rgb(0.5,0.565,0.565) id:paleoarchean value:rgb(0.6,0.592,0.569) id:mesoarchean value:rgb(0.698,0.65,0.6) id:neoarchean value:rgb(0.796,0.804,0.784) id:ediacaran value:rgb(0.918,0.847,0.737) id:cryogenian value:rgb(0.863,0.671,0.667) id:tonian value:rgb(0.796,0.643,0.424) id:stratherian value:rgb(1,1,0.8) # light yellow id:calymmian value:rgb(1,1,0.8) # light yellow id:orosirian value:rgb(1,1,0.8) # light yellow id:rhyacian value:rgb(1,1,0.8) # light yellow id:siderian value:rgb(1,1,0.8) # light yellow id:ectasian value:rgb(1,1,0.8) # light yellow id:stenian value:rgb(1,1,0.8) # light yellow id:cenozoic value:rgb(1,1,0) id:mesozoic value:rgb(0.5,0.6784,0.3176) id:paleozoic value:rgb(0.5,0.7098,0.835) id:phanerozoic value:rgb(0.7019,0.886,0.819) id:proterozoic value:rgb(0.8,0.85,0.568) id:archean value:rgb(0.6,0.6784,0.6745) id:hadean value:rgb(0.4,0.4,0.4) id:black value:black id:white value:white Period = from:-4567.17 till:0 TimeAxis = orientation:horizontal ScaleMajor = unit:year increment:500 start:-4500 ScaleMinor = unit:year increment:100 start:-4500 Define $markred = text:"*" textcolor:red shift:(0,3) fontsize:10 PlotData= align:center textcolor:black fontsize:8 mark:(line,black) width:25 shift:(0,-5) bar:eon at: 0 align:right $markred at: -542 align:left $markred shift:(2,3) from: -542 till: 0 text:Phanerozoic color:phanerozoic from:-2500 till: -542 text:Proterozoic color:proterozoic from:-3800 till: -2500 text:Archean color:archean from: start till: -3800 text:Hadean color:hadean bar:era from: -65.5 till: 0 text:C~z shift:(0,1.5) color:cenozoic from: -251 till: -65.5 text:Meso~zoic shift:(0,1.5) color:mesozoic from: -542 till: -251 text:Paleo~zoic shift:(0,1.5) color:paleozoic from: -1000 till: -542 text:Neoprote-~rozoic shift:(0,1.8) color:neoproterozoic from:-1600 till: -1000 text:Mesoproterozoic color:mesoproterozoic from:-2500 till: -1600 text:Paleoproterozoic color:paleoproterozoic from:-2800 till: -2500 text:Neo-~archean shift:(0,1.5) color:neoarchean from:-3200 till: -2800 text:Meso-~archean shift:(0,1.5) color:mesoarchean from:-3600 till: -3200 text:Paleo-~archean shift:(0,1.5) color:paleoarchean from:-3800 till: -3600 text:Eoar-~chean shift:(0,0.5) color:eoarchean fontsize:6 from:start till: -3800 color:white bar:period fontsize:6 from: -23.03 till: 0 color:neogene from: -65.5 till: -23.03 color:paleogene from: -145.5 till: -65.5 color:cretaceous from: -199.6 till: -145.5 color:jurassic from: -251 till: -199.6 color:triassic from: -299 till: -251 color:permian from: -359.2 till: -299 color:carboniferous from: -416 till: -359.2 color:devonian from: -443.7 till: -416 color:silurian from: -488.3 till: -443.7 color:ordovician from: -542 till: -488.3 color:cambrian from: -630 till: -542 text:Ed. color:ediacaran from: -850 till: -630 text:Cryo-~genian color:cryogenian shift:(0,0.5) from: -1000 till: -850 text:Ton-~ian color:tonian shift:(0,0.5) from: -1200 till: -1000 text:Ste-~nian color:mesoproterozoic shift:(0,0.5) from: -1400 till: -1200 text:Ect-~asian color:mesoproterozoic shift:(0,0.5) from: -1600 till: -1400 text:Calym-~mian color:mesoproterozoic shift:(0,0.5) from: -1800 till: -1600 text:Stath-~erian color:paleoproterozoic shift:(0,0.5) from: -2050 till: -1800 text:Oro-~sirian color:paleoproterozoic shift:(0,0.5) from: -2300 till: -2050 text:Rhy-~acian color:paleoproterozoic shift:(0,0.5) from: -2500 till: -2300 text:Sid-~erian color:paleoproterozoic shift:(0,0.5) from: start till: -2500 color:white Greek root The English word eon is derived from the Koine Greek word aion. Although a proposal was made in 1957 to define an aeon to be a unit of time equal to one billion years (1 Ga), the idea was not approved as a unit of scientific measure and is seldom used for a specific period of time. Its more common usage is for any long, indefinite, period of time. The origin is from the Greek root "aion" for "age" or "life force." A similar Latin word "aevum" for age is still present in words such as Longevity and medieval. http://www.pballew.net/arithm16.html See also Geologic time scale References * International Stratigraphic Chart from the International Commission on Stratigraphy * USA National Park Service * Washington State University * Web Geological Time Machine * Math Words - An alphebetical index Category:Geologic time scale Category:Units of time